The reaction of the hydrolysis of compound sugars in liquid medium, generally an aqueous medium, in the presence of an acidic catalyst, is known. This reaction, sometimes known as "sugar inversion reaction" makes it possible normally to obtain an "invert sugar", that is to say a mixture of simple sugars, in particular of hexoses. In particular, if one starts from a solution of saccharose, one obtains an invert sugar formed of a mixture of fructose and glucose.
The article "Les sucres invertis" (Invert sugars) by BUSSIERE et al. in the IAA, July/August 1990, pages 645 to 649, already describes the hydrolysis reaction in the presence of ion exchange resins as heterogenous acidic catalysts. In order to avoid the formation of coloring substances, it has been believed up to now that it is preferable to operate at low temperature (on the order of 30.degree. to 35.degree. C.). In particular, in the case of the hexoses, it is convenient to avoid the formation of hydroxymethylfurfural (HMF) and of its derivatives or intermediates (polymers, humins, levulinic or formic acid, etc). Now, the actions resulting in the formation of HMF and its derivatives are also catalyzed in acid medium and favored by an elevated temperature, in particular, a temperature above 50.degree. C. Furthermore, the use of an ion-exchange resin as acidic catalyst prohibits high temperatures.
It is to be noted in this connection that if it is desired actually to obtain a mixture of simple sugars, it is advisable to see to it that the reaction stops at the hydrolysis stage, without going beyond it to the degradation of the sugars and the formation of derivative products such as polyols, polymers, acids, HMF, or the like.
Up to now, it has therefore been preferred to limit the conversion rate of the compound sugar in order to avoid the formation of undesirable by-products. Thus, the known hydrolysis reactions do not make it possible, under economically profitable conditions, to obtain pure solutions of fully converted simple catalysts, that is to say without compound sugar or derivative products.
Furthermore, at the low temperatures used up to now with the ion-exchange resins, the conversion of the compound sugar is slight and the reaction requires considerable time, conventionally on the order of 12 to 48 hours.
Furthermore, the compound sugars have a high viscosity at these temperatures. It is now possible in actual use to employ a starting solution which is strongly concentrated in compound sugar, and a concentration step must be provided after the hydrolysis.
Similarly, the use of ion-exchange resins results in numerous drawbacks from an industrial standpoint, such as the formation of waste, difficulties in regeneration, and a high cost of production.
Thus, FR-A-1526 029 describes a process of decolorizing juice, syrup, molasses and even sugar mill waste permitting transforming totally or in part the saccharose comprising a step of decolorizing by ion exchange resins, with partial or total hydrolysis, a step of purification and decoloration by ion exchange resins and a step of concentration. The starting products being highly loaded with cations and anions, the resins are rapidly saturated, so that the process cannot find practical application in industry. Moreover, this process is in several steps, and requires a low inlet concentration (20 to 30 Brix).
For all of these reasons, the known processes, such as the one described in the aforementioned article, have known only limited industrial development and do not make it possible to produce invert sugars at low cost.